LCDs require a backlight. For full color LCDs, the backlight is a white light. The white point of the white light is typically designated by the LCD manufacturer and may be different for different applications. The white point is specified as a temperature. One application of an LCD is a television or computer monitor.
Common white light backlights are created by either a fluorescent bulb or a combination of red, green, and blue LEDs.
For medium and large backlights, multiple LEDs of each color are used. All LEDs of one color on a printed circuit board (PCB) are typically connected in series. Generally, in backlights, external current drivers are used, each driving one string of red, green, or blue LEDs.
The amount of current through an LED controls the brightness. The most straightforward way to control the current of an LED driver, as shown in FIG. 1, is to measure the voltage across a current sense resistor 6 in series with the LEDs 8 (represented by a single LED) and compare this voltage, using a differential amplifier 10, to an accurate reference voltage Vref. The feedback loop automatically adjusts the output of the driver to cause the sense voltage to match the reference voltage.
An alternative is to use a linear current source with a separate voltage source, although this method is not commonly used for backlighting. FIG. 2 illustrates a series string of LEDs of a single color being driven by a linear current source 13. In a linear current source, the current is controlled by controlling the conductance of a pass transistor 14. The current source may be controlled to generate a preset current by detecting the current with a current detector 16 and applying the detected current as a feedback signal to a control circuit 18 to maintain the current at a preset level.
As shown in FIG. 3, the current detector may comprise a low value resistor 20 with a differential amplifier 22 measuring the voltage drop across the resistor 20. The control circuit 18 may be a second differential amplifier 24 that receives the current feedback signal and receives a settable reference voltage Vref. The amplifier 24 controls transistor 14 to maintain the output of amplifier 22 at Vref. Vref may be set by selecting a resistance in a voltage divider between a fixed voltage and ground or may be set by other means. An enable signal (EN) turns the amplifier 24 (and the transistor 14) on or off. Many other embodiments are possible.
Whichever of the above drivers are used, the common practice for backlighting is to control the current through the LED strings externally, i.e., not on the PCB. Furthermore to limit the number of drivers, multiple boards of LEDs are placed in series, with the consequence that the strings of equal color have the same drive current on multiple boards. Without further precautions, this will lead to large variations in brightness and color, due to the inherent variations in color and brightness between individual LEDs.
Generally, color and brightness uniformity is improved by selecting LEDs for specific locations on the boards. This is done by testing the individual LEDs, then “binning” each LED according to its characteristics, and then placing LEDs on the boards in accordance with precalculated bin patterns. Additionally, the boards' light emitting characteristics are measured after placing of the LED, and the boards are combined such that only boards with closely matching white points are used in a single backlight. This process is called grading. The process of using bin patterns and grading in an attempt to create boards with uniform light characteristics and achieve a target white point is costly and time consuming. Furthermore, variations within a PCB are not fully suppressed.